[LeetCode] Problem 67 - Add Binary

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Given two binary strings, return their sum (also a binary string).

The input strings are both non-empty and contains only characters 1 or 0.

Example

No.1

Input: a = “11”, b = “1”

Output: “100”

No.2

Input: a = “1010”, b = “1011”

Output: “10101”

Code

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public String addBinary(String a, String b) {
    int carry = 0;
    StringBuffer result = new StringBuffer();

    for (int i = a.length() - 1, j = b.length() - 1; i >= 0 || j >= 0; i--, j--) {
        int sum = carry;

        if (i >= 0)
            sum += a.charAt(i) - '0';
        if (j >= 0)
            sum += b.charAt(j) - '0';
        
        carry = sum / 2;
        result.insert(0, sum % 2);
    }

    if (carry > 0)
        result.insert(0, carry);
    
    return result.toString();
}